River and Stream Flow Monitoring Methods
Ecology scientists called hydrologists measure the width, depth, and speed of water in rivers and streams to compute streamflow. Standard Operating Procedure for Measuring and Calculating Stream Discharge - EAP056 (PDF, 695 K) is the official document describing streamflow methods in great detail. A basic description of the way streamflow is calculated can be found below.
Streamflow is how much water, how fast and how high
Streamflow is the amount of water passing a specific point in a stream at a given time. It is sometimes called discharge, and it is often expressed in cubic feet per second, or cfs.
Water level in a stream is its stage
To speak scientifically about the water level in a river or stream, a hydrologist talks about a stream's "stage." The stage of a stream is its height or depth in relation to a fixed measuring point in the stream.
Stream stage predicts streamflow
A "rating curve" is a mathematical tool used to predict streamflow based on stage height. Hydrologists develop a rating curve for a site after recording a series of area and velocity measurements of the stream at different stages. This allows them to predict streamflow with only stage height measurements. This rating curve is different for each gaging site and can change. Some stream channels are more stable than others, so some streams have more- or less-stable rating curves. Flow predictions based on more-stable rating curves are generally considered more reliable.
Water pressure shows stage height
Ecology's gaging stations are generally equipped with instruments that measure and record a stream's stage by measuring water pressure. A gas-bubbler system or an electronic transducer at a known location below the water measures the pressure of the water above it. This measured pressure determines the stage of the water in the stream. Some stations use radar equipment to determine stage.
Stage records transmit every 15 minutes
A data logger records stage measurements every 15 minutes and sends this information to Ecology via satellite. After the rating curve is applied to the stage record, the public can access streamflow information directly by going to each site on its Flow Monitoring Network page.
Hydrologists confirm streamflow in the field
The scientists visit streamflow sites about once every six weeks to measure flow and to confirm predictions and/or further develop the rating curve. They also may perform maintenance on the equipment. They confirm the stage to maintain calibration of the data logger using a physical index that may include a staff gage, usually one or more porcelain enameled steel plates mounted to a secure structure, a wire-weight gage attached to a bridge, or a laser level mounted at a fixed elevation. Most stations are also equipped with secondary or backup gage indices.
Additionally, they measure actual streamflow with hydroacoustic equipment. This can include an Acoustic Doppler Velocimeter, called an ADV, used in smaller streams, that bounces a pulse of sound against moving particles in the stream to measure velocity. The Acoustic Doppler Current Profiler or ADCP is moved across larger streams on a raft and can also determine stream depth. The change in the frequency of reflected sound (the "Doppler Effect") allows these instruments to calculate velocity and streamflow very accurately. Further, hydrologists periodically confirm all reference points with survey equipment.
Findings from each visit to a streamflow gaging site, including any changes in the rating curve, are reported in the Technical Notes for that site, published with the streamflow site data and available in a tab on each site's Flow Monitoring Network page.
Flow Monitoring Network will take you to nearly real-time stage and streamflow records.
Using River and Streamflow Data describes how to understand the information on the Flow Monitoring Network pages.
Surface-Water Procedures and Policies Training Course is a detailed USGS training course (SW1660) on streamflow gaging.
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